Preheat the broiler. Lightly butter the bottom of a shallow baking dish. Lay the pineapple slices in a single layer in the dish. Sprinkle the tops with the brown sugar and cinnamon. Drizzle the melted butter on top. Broil until the sugar melts and is bubbly. Sprinkle the coconut on top and broil again for a few seconds. The slices are ready when the coconut is toasted to a golden brown. Serve the slices warm, by themselves or with ice cream. You may also place them on slices of poundcake, drizzling the pan juices on top before serving.